• S K Sharma

      Articles written in Pramana – Journal of Physics

    • Kinetic theory of parametric excitation of acoustic waves in piezoelectric semiconductors

      A K Sundaram P K Kaw S K Sharma

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      Using a kinetic description for electrons and the usual equation of motion for lattice displacement we have derived a general dispersion relation for acoustic waves in a piezoelectric semiconductor, in the presence of a strong high frequency electric field oscillating near the electron plasma frequency. Earlier hydrodynamic results valid fore≪1 (wherek is the wave number of the acoustic wave andλethe electron mean free path) are rederived as a special case. Fore≫1, two instability branches are discovered and magnitudes of the threshold electric field required to drive the acoustic wave unstable in each case, are obtained.

    • Inelastic interactions of 9.38 GeV/c deuteron in emulsion

      S K Badyal V K Gupta G L Kaul Balvinder Kour Y Prakash N K Rao S K Sharma Gian Singh

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      Characteristics of the inelastic interactions of 9.38 GeV/c deuterons with nuclear emulsion nuclei have been studied. These have been compared with nucleon-nucleus interactions at a corresponding momentum. The probability of nucleon stripping in deuteron-nucleus interactions has been observed to be 0.5. The charged particle multiplicity in deuteron-nucleus interactions exhibitA-dependence of the typeAα with α=0.08. The experimental data disagree with KNO scaling.

    • Variation of average charged particle multiplicity inp-nucleus interactions with energy and the two component description of particle production at high energies

      G L Kaul S K Badyal I K Daftari V K Gupta B Kour L K Mangotra Y Prakash N K Rao S K Sharma Gian Singh

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      Experimental data on average shower particle multiplicity (〈Ns〉) accumulated onp-nucleus interactions in the wide momentum region of 7.1–8000 GeV/c is investigated. It is observed that 〈Ns〉 is represented exceedingly well as a function of (vvS). There are two physical processes which represent the experimental data reasonably well in the two momentum regionsviz 7.1–67.9 GeV/c and 67.9–8000 GeV/c. 〈Ns〉=a(vS)/a+b fits the data in the low momentum region, whereas 〈Ns〉=a +b ln (vvS) fits the experimental data in the high momentum region. The two physical processes are unified and represented by a single equation which is shown to be the consequence of two component theory and collective models.

    • Pressure-induced amorphization of Gd2(MoO4)3: A high pressure Raman investigation

      A Jayaraman S K Sharma S Y Wang

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      High pressure Raman spectroscopic studies on Gd2(MoO4)3(GMO) have been carried out at ambient temperature in the diamond cell to 10 GPa hydrostatic pressure. These experiments have revealed pressure-induced phase transitions in GMO near 2 GPa and 6.0 GPa. The first transition is from Pba2(β′) phase to another undetermined crystalline phase, designated as phase II, and the second transition is to an amorphized state. On releasing pressure there is a partial reversion to the crystalline state. The Raman data indicate that the amorphization is due to disordering of the MoO4 tetrahedral units. Further, it is inferred from the nature of the Raman bands in the amorphized material that the Mo-O bond lengths and bond angles have a range of values, instead of a few set values. The results of the present study as well as previous high pressure-high temperature quenching experiments strongly support that pressure-induced amorphization in GMO is a consequence of the kinetically impededβ toα phase transition. The system in frustration becomes disordered. The rare earth trimolybdates crystallizing in theβ′ structure are all expected to undergo similar pressure-induced amorphization.

    • Further studies on Ag/BPSCCO tapes using low purity materials

      S R Shukla Y S Reddy N Kumar S K Sharma R G Sharma

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      Transmission electron microscopic (TEM) studies are reported on Ag-clad Bi1.7 Pb0.4Sr1.8Ca2Cu3.5Ox tapes prepared by using low purity (98–99%) commercial grade materials. The self-fieldJc values of these tapes viz. 6.14 × 103 A.cm−2 at 77 K and 1.4 × 105 A.cm−2 at 4.2 K, reported in an earlier publication, were significantly higher than the correspondingJc values in tapes prepared with high purity (99.99%) materials. The TEM pictures on the low purity core material of the tapes reveal the presence of stacking faults and the intergrowth of the 2212 and 2223 phases which could be acting as flux pinning sites and responsible for enhancedJc values. These defects can perhaps be traced back to the presence of 60 ppm iron in the low purity CuO as revealed by atomic absorption analysis reported earlier.

    • Pressure-induced structural and electronic transition in KTb(MoO4)2 through Raman and optical studies

      A Jayaraman S K Sharma S Y Wang S R Shieh L C Ming S-W Cheong

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      Raman and optical absorption studies under pressure have been conducted on KTb(MoO4)2 up to 35.5 GPa. A phase transformation occurs at 2.7 GPa when the crystal is pressurized at ambient temperature in a hydrostatic pressure medium. The sample changes to a deep yellow color at the transition and visibly contracts in theα-axis direction. The color shifts to red on further pressure increase. The Raman spectral features and the X-ray powder pattern change abruptly at the transition indicating a structural change. The pressure-induced transition appears to be a property of the layer-type alkali rare earth dimolybdates. However, the color change at the transition in KTb(MoO4)2 is rather unusual and is attributed to a valence change in Tb initiated by the structural transition and consequent intervalence charge transfer between Tb and Mo.In situ high pressure X-ray diffraction data suggest that phase II could be orthorhombic with a unit cell having 3 to 4% smaller volume than that of phase I.

    • Operational characteristics of dual gain single cavity Nd:YVO4 laser

      Pranab K Mukhopadhyay Jogy George S K Sharma P K Gupta T P S Nathan

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      Operational characteristics of a dual gain single cavity Nd:YVO4 laser have been investigated. With semiconductor diode laser pump power of 2 W, 800 mW output was obtained with a slope efficiency of 49%. Further, by changing the relative orientation of the two crystals the polarization characteristics of the output could be varied. In particular by keeping the two Nd:YVO4 crystals with their c-axes orthogonal to each other and adjusting the gain of the crystals so that both operate at approximately the same power level, completely unpolarized beams could be obtained.

    • Brief report: Volume dependence of Grüneisen parameter for solids under extreme compression

      SANJAY KUMAR S K SHARMA O P PANDEY

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      The Nie expression is amended in such a way that the expression follows the infinite pressure behaviour, i.e., P → ∞or V → 0. A new empirical relationship is developed to predict the values of volume dependence of Grüneisen parameter. NaCl and ε-Fe have been employed to test the suitability of the expression.The results obtained reveal that the relationship is reliable as there is a good agreement between the calculated and the experimental data

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